Apparently VS 2008 does not allow setting a breakpoint just on the setter of an auto-property.

I.e. if I define an auto-property like this:

public int CurrentFramesize
{
get;
protected set;
}

and then try to set a breakpoint on the setter line, the whole auto-property turns breakpoint-red.

This works just fine for normal properties, so any idea why auto-properties get this special (restrictive) treatment? Are they more than just syntactic sugar to normal properties with a hidden backing field?

No you can't set a break point on them, but then what would you check? The variable for storage of the auto-property is only assigned at runtime and as such there isn't a variable for the debugger to show/access.